The present invention relates generally to free machining steels containing bismuth and more particularly to a bismuth-containing cast steel shape in which the opportunity for the bismuth to function as a liquid metal embrittler is increased.
In the machining of steel, a cutting tool is applied to the surface of the steel, and either the steel or the tool is moved relative to the other to effect a cutting of the steel by the tool. This forms chips of steel which are removed from the steel during the machining operation. Chip formation is related to the formation and propagation of microcracks in the steel.
More specifically, during machining, a force is applied to the steel at a location where the cutting edge of the tool contacts the steel, and this force causes microcracks to form in the steel. These microcracks may originate at inclusions in the steel, or these microcracks may extend into the steel from the location where the steel is contacted by the cutting edge of the tool to an inner-most tip of the microcrack. These microcracks generally proceed along grain boundaries or inter-phase boundaries in the steel. To propagate these microcracks requires the expenditure of energy during the machining operation. The smaller the expenditure of energy required to propagate the microcrack, the easier it is to machine the steel, and therefore, the better the machinability of the steel.
During machining, the temperature of the steel in the vicinity of a microcrack is raised by the heat generated in the machining operation. The temperature increase of the steel, due to the machining operation, is highest at the cutting edge of the machining tool and decreases as the distance from the cutting edge increases.
If a liquid metal embrittler is present at or in the vicinity of the innermost tip of a microcrack, the energy required to propagate the microcrack is lowered. A liquid metal embrittler is a metal or alloy which has a relatively low melting point, so that it is liquid at the temperature prevailing at the tip of the microcrack during machining, and which also has a relatively low surface-free energy value near its melting point so as to impart to the liquid metal embrittler the ability to wet a relatively large surface area along grain boundaries or interphase boundaries. The lower the surface free energy value (or surface tension), the greater the surface area coverage of the liquid metal embrittler. Normally, the surface free energy value of a liquid metal embrittler rapidly decreases (and thus its wetting ability rapidly increases) at the melting point of the liquid metal embrittler.
When a microcrack is initially propagated in the vicinity of an inclusion containing a liquid metal embrittler, and the temperature at the location of that inclusion has been raised sufficiently to liquify the liquid metal embrittler, there is an almost immediate transport of liquid metal embrittler to the tip of the microcrack. This transport proceeds along grain boundaries, phase boundaries or the like. The liquid metal embrittler thus transported may be a layer only a few atoms thick, but that is enough to perform its intended function as a liquid metal embrittler at the microcrack.
Because the ability of a liquid metal embrittler to function as such is directly related to the immediate transport thereof to the tip of a microcrack, anything which enhances the likelihood of immediate transport to the tip of the microcrack is desirable.
The lower the melting point of the liquid metal embrittler and the stronger its tendency to wet the steel grain boundaries or interphase boundaries, the farther away from the tool cutting edge are regions of the steel embrittled for easier fracture.
It has been conventional to add sulfur to steel to improve machinability. Sulfur combines with manganese to form manganese sulfide inclusions in the steel. The manganese content is typically about two and one-half times the sulfur content of the steel to assure that the sulfur combines with the manganese rather than iron thereby avoiding a hot rolling defect known as hot shortness. Manganese can strengthen the steel by a mechanism known as solid solution strengthening. The manganese which combines with the sulfur is not available to strengthen the steel.
Elements which have been added to steel to increase its machinability include lead, tellurium, bismuth and sulfur, all of which are present as inclusions in the microstructure of the steel. Heretofore it has been considered undersirable for the microstructure to contain fine-sized inclusions of machinability increasing elements. For example, with respect to manganese sulfide inclusions, 15 microns is considered an optimum size, with inclusion sizes being generally in the range 10-30 microns, and less than 5 microns is considered bad.